Yes, there are reasons. One of them is fasting as already mentioned. Historically according to the Typikon tradition, a vesperal Liturgy is offered at 5 different times of the year. 4 times of the year, it is Liturgy combined with St. Basil Liturgy on the eve of the feast, namely the eves of Nativity, Theophany, Holy Friday, and Pascha (the "eve" of the latter two being Holy Thursday and Holy Saturday afternoon/evening). One time out of the year, Annunciation, it is Vespers with the Liturgy of St. John Chrysostom when falling on a weekday in Lent, and is held in the afternoon following the morning of the feast itself. However, in modern Greek practice, for at least 150 years, it has been held without Vespers in the morning. Even though an "eve" (before the morning) Liturgy is in conjunction with St. Basil traditionally, and only the evening following is in conjunction with St. John Chrysostom, the Antiochian Archdiocese has printed texts for the eve (before) of Vesperal Liturgies of St. John Chrysostom. It is ultimately up to the priest with the approval of the Bishop. I myself get no more people for the eve of the feast than I do for the morning (although it is different people), so I keep it as called for.